1. Field of the Invention
In one of its aspects, the present invention relates to a process for preparing a functionalised polyolefin, preferably via free radical chemistry. In another one of its aspects, the present invention relates to a functionalised polyolefin. In yet another one of its aspects, the present invention relates to a moisture curable polyolefin resin, particularly such a resin that bonds covalently to siliceous particulate material (e.g., fibers).
2. Description of the Prior Art
Polyolefin composites and blends have many applications. Of particular commercial interest are composites and blends of polypropylene, ethylene-propylene copolymers and polyisobutylene. Polypropylene, for example, is the material of choice in many packaging and automotive applications.
In recent years, there has been interest in functionalizing polymers with a view to improving and/or optimizing the chemical and/or physical properties of the base polymer with respect to its intended use.
The introduction of reactive and/or polar functional groups to polyolefins (a process referred to herein as “modifying”) can greatly enhance the chemical and physical properties of the resulting polymer and its compounds. It is therefore advantageous to have cost-effective methods to introduce moieties such as anhydride, ester, amide, urethane, hydroxyl, amino, ether, silyl-ether, epoxide and alkoxysilane groups to conventional polyolefins.
Radical-mediated polymer modification using maleic anhydride, vinylsilanes and acrylate monomers is a conventional method for preparing functionalised commodity materials. However, these radical-mediated modifications typically have some adverse consequences particularly with regard to higher molecular weight polyolefins.
Specifically, when the polyolefin is polypropylene, polyisobutylene or the like, the free-radical chemistry used to graft these types of functionality can result in significant reductions in the molecular weight of the polyolefins. The resulting modified polyolefin product can have reduced viscosity and relatively poor mechanical properties, rendering it unsuitable for many industrial applications.
For example, free-radical graft-modified polypropylene and/or polyisobutylene resins have low viscosities, and are unsuitable for many industrial applications. While commercially available graft modified polypropylene resins are used as compatibilization agents for polymer blends and composites, the low molecular weight of these materials (a corollary of the fragmentation process) precludes their use in many consumer goods. Additionally, the mechanical properties of the resin also can be significantly diminished by the grafting process.
Thus, conventional approaches of radical-mediated modification of polyolefins result in alteration of the molecular weight of the polymer (i.e., decrease in molecular weight for polyolefins). This leads to a correspondingly altered viscosity of the polymer product which significantly reduces the scope of useful applications thereof.
Known strategies to minimize the effects of or degree of fragmentation and radical combination are: careful selection of starting resin and careful control of reagent concentrations. However, these methods, whether alone or in combination, do not limit the extent of fragmentation to a satisfactory level.
It is known to modify a polybutene containing a terminal double bond with an organic thiol. For example, European patent 0,342,792B teaches reacting a polybutene containing carbon-carbon double bonds with an organic thiol to form a polybutene having a thioether function. The patent teaches performing this reaction under free-radical conditions, and that it may be performed in the absence of solvent. However, the reaction taught is limited to polybutenes having a molecular weight ranging from 200 to 10,000, preferably from 400 to 2500. The description teaches that use of a polybutene of too low a molecular weight results in an addition product of relatively high volatility, while a polybutene of too high a molecular weight results in an addition reaction of low yield.
“Functional Polypropylene Blend Compatibilizers”, Markomol. Chem, Macromol. Symp. 48/49, 317-332 (1991) [Mülhaupt et al. (Mülhaupt)] teaches a range of monofunctional polypropylenes containing functional endgroups, including sulfides, derived from mono-olefin-terminated polypropylene. Mülhaupt teaches a process of forming sulfide-terminated polypropylene having average molecular weight of about 900. However, Mülhaupt notes that, while free radical induced addition reactions have been carried out successfully using thiol compounds, the double bond conversion is frequently incomplete. Thus, the process taught by Mülhaupt fails to achieve 100% gelation and quantitative binding of the modified polymer to a siliceous particulate material.
While European patent 0,342,792B and Mülhaupt teach reacting low molecular weight polybutene or polypropylene with a thiol, this method has not been applied to higher molecular weight polyolefins such as polyethylene. Further, neither of these references teaches or suggests a functionalized polyolefin that may be moisture cured.
Higher molecular weight polymer resins are advantageously capable of being moisture-cured. Practically, to achieve a suitable graft content, a minimum molecular-weight of about 10,000 is needed so that the resulting polymer resin is capable of being moisture-cured. For the purpose of various applications, these moisture-curable polymer resins are advantageously bound to siliceous fillers. Gelling of these polymer resins containing siliceous fillers similarly requires a minimum molecular-weight of about 10,000. However, as a result of the molecular weight degradation associated with known radical modification of polypropylene (discussed above), the production of a moisture-curable polypropylene having a practical molecular weight based on such resins is not known.
Thus, despite the advances made in the art, there exists a need for functionalised polyolefin derivatives of high molecular weight polymers that can be produced via radical chemistry such that the molecular weight of the functionalised polyolefin derivative does not significantly change (via degradation or increase) during production thereof. More particularly, it would be highly desirable to be able to modify or functionalize a polyolefin without a consequential significant alteration of the molecular weight of the resulting polymer. Specifically, it would be highly desirable to be able to modify or functionalize a polyolefin without a consequential significant reduction in the molecular weight of the resulting polymer.